Line of draft
What is a line of draft? No it is not a queue for more beer on tap. It refers to the line of leverage from the point of where your horse starts pushing against his collar to were the trace is attached to the cart or carriage. This angle is equivalent to a person pushing a heavily loaded wheel barrow. That person will, depending upon their own strength and conformation, and how the load in the barrow is balanced, raise or lower the handles to most efficiently and comfortably move the load along.
This aiding and abetting efficiency and comfort is why we care about line of draft in our cart/carriage horses. The horse in harness cannot adjust his harness or balance the load himself; his observant driver needs to do this for him. The more you keep your harness animal comfortable, the happier and easier he will work.

Line of draft with a Breast Collar - maximum efficiency is if the line is straight back and level.

Line of draft on a Neck Collar. The goal is to have the weight bearing collar have even pressure completely around the neck. This is achieved by taking into consideration the angle of the shoulders the collar rests on, and the angle of the line pulling down on the collar. These lines can be tweaked by where the traces are attached to the hames themselves depending upon if your carriage attachment is higher or lower.
An article on my Fjord Pair turnout may be found at: http://bluemountainfarm.net/fjord10.htm
An awesome harness resource is:

Understanding Harness by Barb Lee
